Thermocouple Construction and Types
A thermocouple consists of two different types of metals joined together at one end (hot junction) and terminated at another end (cold junction). When the hot junction is heated or cooled, a voltage created can be correlated back to the temperature.
Theoretically, any pair of dissimilar materials can be used for the construction of a thermocouple. But practically, only a few materials have found useful for measuring temperature. The following table shows common types of thermocouple materials with their temperature range.
|Type||Positive Lead||Negative Lead||Temperature Range|
|R||Platinum-Rhodium (87% Pt,13% Rh)||Platinum||0 – 1500 oC|
|S||Platinum-Rhodium (90% Pt, 10% Rh)||Platinum||0 – 1500 oC|
|K||Chromel (90%Ni, 10% Cr)||Alumel (Ni94Al2Mn3Si)||-200 – 1300 oC|
|E||Chromel (90%Ni, 10% Cr)||Constantan (57% Cu, 43% Ni)||-200 – 1000 oC|
|T||Copper||Constantan (57% Cu, 43% Ni)||-200 – 350 oC|
|J||Iron||Constantan (57% Cu, 43% Ni)||-150 – 750 oC|
Thermocouple Working Principle
The thermocouple works on the principle of see back effect. Thomas Johan Seeback discovered that temperature difference(thermal energy) can produce electrical energy. In a thermocouple, two conductors of different type of metals are connected forming two common junctions. When these two junctions are exposed to two different temperatures, a net thermal emf is produced. The value also depends on the materials used and is proportional to the temperature difference between hot and cold junctions.
The output of a thermocouple can be measured directly by a millivoltmeter. It can be also measured with the help of a dc potentiometer or using amplifiers with an output device.
Advantages of Thermocouple
- Cheaper than resistance thermometers.
- Can measure rapid changes in temperature.
- Wide temperature range.
- Offers good reproducibility.
- Convenient for measuring the temperature at one particular point.
Dis-advantages of Thermocouple
- Lower accuracy.
- The induced emf-temperature characteristics are non-linear.
- Signal amplification is required in many applications.